1. Field of the Invention
This invention relates generally to computer memory devices and more particularly to memory devices that must be periodically refreshed to avoid losing data.
2. Description of the Related Art
Certain memory devices can maintain the information stored in the memory indefinitely, even when the power to the memory device is turned off. These memory devices are known as non-volatile memory devices. Some examples of non-volatile memory devices include magnetic random access memories (MRAMs), erasable programmable read only memories (EPROMs), and variations thereof.
Other memory devices require power to maintain the information stored in the memory. These memory devices, which are known as volatile memory devices, must be periodically refreshed to avoid losing data. One common example of a volatile memory device is a dynamic random access memory (DRAM), wherein voltages stored in capacitors represent digital bits of information. Because the voltage stored in a capacitor dissipates over time, the capacitors of a DRAM must be periodically re-charged to maintain the information stored in the DRAM.
Conventional volatile memory devices include a plurality of individual memory cells configured in an array, which typically comprises a configuration of intersecting rows and columns. To maintain the information stored in a conventional volatile memory device, each memory cell in the array is typically refreshed at least several times per second. For example, in some conventional DRAMs, each memory cell must be refreshed about once every 64 milliseconds to avoid losing the information stored in the DRAM. Accordingly, conventional volatile memory devices typically include refresh circuitry to ensure that every memory cell is refreshed at least as often as necessary to avoid losing data, which is commonly at least several times per second.